Propagation Of Magnetoacoustic Waves Research Articles

Propagation of nonlinear magnetoacoustic waves in a compressible medium of finite electrical conductivity

We study the one-dimensional propagation of weakly nonlinear waves in a compressible medium of finite electrical conductivity subjected to the action of a magnetic field. We obtain evolution equations that describe the wave processes under small and finite magnetic Reynolds numbers. It is shown that in a medium of finite conductivity the evolution of perturbations in a fluid is described by the modified Burger's equation. We find the stationary and automodel solution of this equation and use them as the basis for analyzing the influence of effects of electrical conductivity on the structure of weak shock waves.

Propagation of magnetoacoustic waves in the solar atmosphere with random inhomogeneities of density and magnetic fields

The propagation of acoustic waves in a plasma containing an ensemble of tightly settled bundles of magnetic flux tubes is studied for the case where all the parameters of the medium (magnetic field, plasma density, temperature, etc.) change by an order of unity over a length small compared to the acoustic wavelength. Such a model of random and strong inhomogeneities can correspond, for example, to sunspots in the solar atmosphere, to plage regions, and to other astrophysical objects, where domains of different size with different plasma parameters compose a dense conglomerate. The theoretical procedure is described that allows one to obtain the average equations containing the nonlinearity of a wave, dispersion properties of a system, and dissipative effects.

The electrodynamics of railgun plasma armatures

A single-fluid, one-dimensional, time-dependent model of an unconfined plasma armature is discussed. The temporal evolution of the pressure and current density distributions is shown to be controlled by a combination of magnetic diffusion, shear, ablation, and magnetoacoustic wave propagation within the plasma. Different parameter regimes are explored, and the characteristic time constants and length scales are established for these various physical processes. It is shown that, when the plasma slip-velocity is low, the plasma may fragment or disperse by virtue of a diffusion-driven instability. Conversely, when ablation is high and the slip-velocity is large, the plasma will form a parasitic current sheet at the tail of the armature, which may detach from the primary plasma. The first of these instabilities is initiated by a decrease in the input current. The second occurs above a certain critical combination of magnetic field strength and ablation coefficient.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Conditions for vertical propagation of magnetoacoustic waves in an isothermal atmosphere

The propagation of magnetoacoustic waves in an isothermal atmosphere with constant gravity and uniform magnetic field of an arbitrary direction is considered. The conditions for the vertical propagation of these waves are determined by characteristic frequencies that arise in the MHD wave equation which is considered here in the limit of low-beta plasma. The obtained steady state solutions to the wave equation describe the behavior of fast and slow magnetoacoustic waves and allow defining the characteristic frequencies that separate domains of wave frequency where the sinusoidal and nonsinusoidal wave solutions are found. The particular problem of dependence of the characteristic frequencies on the density gradient as well as on the strength and direction of the uniform magnetic field are discussed. The obtained results may have important consequences on the behavior of magnetoacoustic waves in stellar atmospheres and may be found useful in studying stellar p-mode oscillations.

Magnetoacoustic wave propagation in paramagnetic insulators exhibiting induced linear magnetoelastic couplings

The propagation of bulk and surface magnetoacoustic waves is studied in paramagnetic materials on the basis of a phenomenological, fully nonlinear, rotationally invariant, thermodynamically admissible theory of the magnetoelasticity of insulators such as rare‐earth compounds. The work is self‐contained and develops from basic nonlinear equations to solve for propagation modes after linearization of both volume and surface equations about a bias state in accordance with the methods of nonlinear continuum mechanics. The initial nonlinear feature of the theory, associated with rotational invariance, allows one to place in evidence the effects of a bias magnetic field on the dynamical properties of the medium. In particular, among the effects thus exhibited are (1) a symmetry breaking favoring the existence of magnetoelastic couplings (biased piezomagnetism), which would not exist in the absence of bias fields, (2) an alteration in the bulk‐wave speeds (direct magnetoacoustic effects), (3) an acoustical linear birefringence, and (4) in addition to the classical Rayleigh mode, creation of an SH magnetoacoustic surface mode akin to the Bleustein–Gulyaev mode of acoustoelectricity, results from adequate symmetry breaking effects of the bias magnetic field. The influence of viscosity and magnetic relaxation is also examined.

Absorption magnetoakustischer Wellen in einem Hoch-Beta-Plasma

Abstract Absorption of magnetoacoustic waves in a high beta plasma The radial propagation and absorption of magnetoacoustic waves (m = 0, kz = 0) in a radially inhomogeneous, high beta plasma column of a theta pinch is investigated both theoretically and experimentally. It is shown, that viscosity is the dominant dissipative mechanism. Power absorption up to 10 MW is obtained, which corresponds to an average heating rate of 5 eV/μsec per particle.

An exact solution to the equation of propagation of magneto-acoustic waves

By assuming a uniform horizontal magnetic field and using Lagrangian coordinates the equations of motion, continuity, and magnetic induction are combined to give the exact nonlinear partial differential equation of propagation of vertically travelling magneto-acoustic waves in an atmosphere of uniform density. By means of an intermediary integral, an exact solution is then derived for the cases of a polytropic gas with the ratio of the principal specific heats equal to 5/3 and 2. The solutions are compared with the classical solution of Riemann which describes pure, finite amplitude sound waves propagating in a uniform atmosphere. The degeneration of a magneto-acoustic wave into a shock is illustrated, and the effect of increasing the permanent magnetic field is found to decrease the time for the creation of the discontinuity.

Observations of Moon‐plasma interactions by orbital and surface experiments

Extensive magnetic field observations together with crucial plasma measurements by the Explorer 35 lunar orbiter and Apollo surface and orbital experiments have established the basic nature of the moon's interaction with the solar wind and interplanetary magnetic field. The effective absorption of the incident solar wind by the moon creates a plasma void or cavity behind the moon. The cavity‐associated magnetic signature is characterized by an enhancement in magnetic field magnitude B within the cavity as compared with the mean level of B in the surrounding interplanetary plasma and dips or decreases in B near the cavity boundaries with the solar wind. The axis of the lunar wake is aberrated from the moon‐sun line by the relative velocity of the solar wind with respect to the moon, and the cross section of the wake is elliptical, reflecting the anisotropic propagation of magnetoacoustic waves in the solar wind. Enhancements in B are often observed just external, i.e., on the solar wind side, of the rarefaction‐associated dips in the field magnitude. These are of perturbation magnitude at the altitude of Explorer 35, whereas at the low height of the subsatellites they are often large field increases or limb compressions that occur either just upstream of or directly above lunar limbs. Particular surface regions of the moon seem more effective than others as a source of the limb compressions; the pertinent regional property is likely to be remanent magnetization. Propagation of the limb disturbances downstream from the source and the efficacy with which a regional property can create a disturbance in the solar wind flow past the moon's limb depend on properties of the plasma and interplanetary magnetic field. The main enhancement of B in the shadow region behind the moon is a more conspicuous feature of the magnetosheath interaction, whereas the field strength dips and the limb compressions become more difficult to identify in the highly variable state of the magnetic field. In the lobes of the geomagnetic tail, remanent lunar magnetic fields are readily detected at the subsatellite altitude. There is a distinct structure associated with the average field magnitude measured by the Apollo 15 subsatellite in the plasma sheet of the geomagnetic tail; B is enhanced over the day side quadrant of the subsatellite orbit. This variation in the average magnitude of the magnetic field may be associated with the drift of plasma sheet particles toward the earth, creating a wake region centered approximately on the day side orbital quadrant of the subsatellite. Alternatively, flux tube depletion earthward of the moon may explain the observed day side enhancement of average magnetic field magnitude. Apollo particle and field measurements on the lunar surface have provided evidence of a regional interaction of the highly conducting solar wind with lunar remanent magnetic fields. Simultaneous Ogo 5, Vela 5, and Apollo 12 spectrometer data of ion density and velocity taken when the Ogo and Vela satellites were in the solar wind and the Apollo spectrometer was on the lunar day side also exposed to solar wind plasma show that the proton velocity is smaller and the proton density is larger at the Apollo site than in the free stream solar wind, a result of proton deceleration at the site by an electric field established via the plasma‐remanent magnetic field interaction. Simultaneous plasma and magnetic field data, from the spectrometer and the lunar surface magnetometer at the Apollo 12 location, show the compression of the local remanent field by large solar wind and magnetosheath plasma dynamic pressures.

The Trapped Magnetoatmospheric Waves

view Abstract Citations (28) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Trapped Magnetoatmospheric Waves Nakagawa, Y. ; Priest, E. R. ; Wellck, R. E. Abstract The trapped (vertically nonpropagating) magnetoatmospheric waves are examined on the basis of local dispersion relations governing the propagation of magnetoacoustic and magnetogravitational waves in an inviscid atmosphere of infinite electrical conductivity permeated by a uniform magnetic field. It is shown from the necessary condition that trapped waves are possible, (1) when the magnetic field is purely horizontal, (2) when the magnetic field is purely vertical, and (3) when the magnetic field has both vertical and horizontal components for waves propagating perpendicular to the magnetic field. Through examinations of the sufficient conditions for each of these three specific circumstances, the domain of trapped waves is determined in the (angular frequency, horizontal wavenumber)-plane. It is shown that for a horizontal magnetic field, most of the magnetoatmospheric waves become trapped, except for the magnetoacoustic waves. It is shown also that for a vertical or a general magnetic field, most of the magnetoatmospheric waves with the frequency below the local Brunt- frequency could become trapped. The physical significance and possible practical application of the results to solar observations are discussed. Subject headings: atmospheres, solar - hydromagnetics - solar atmospheric motions Publication: The Astrophysical Journal Pub Date: September 1973 DOI: 10.1086/152383 Bibcode: 1973ApJ...184..931N full text sources ADS |

Weak Nonlinear Magneto-Acoustic Waves in an Inhomogeneous Plasma

Effect of inhomogeneity in plasma density and magnetic field on the propagation of magneto-acoustic waves in a cold collision-free plasma is investigated. It is shown, by using a nonlinear perturbation method, that the basic system of equations governing one-dimensional plane waves with small but finite amplitude can be reduced to a modified form of the Korteweg-de Vries equation, from which we can expect damping, growing or splitting of the waves.

Magneto-acoustic-wave traps created by ring currents

The paper deals with the propagation of fast magneto-acoustic waves in a low-density plasma (pB ≫ p) situated in a magnetic field created by two parallel or anti-parallel currents.The transparence of different regions of the magnetic trap for magneto-acoustic waves is studied. The existence of ring-like traps for magneto-acoustic waves is stated.

Propagation of magnetoacoustic waves inside magnetic plasma traps

The paper deals with the propagation of magnetoacoustic waves in plasma traps by making use of the methods of geometrical optics. It shows that annular traps whose axis is the symmetry axis and in which magnetoacoustic waves can be concentrated, exist in the magnetic plasma traps such as are used in experiments aiming at the achievement of nuclear reactions. These traps are the domains enclosed by the surfaces U/r = const. or U = const., where U is the Alfvén velocity and r the cylindrical co-ordinate, depending on the symmetry of the problem. The structure and fundamental properties of these domains are studied for five special models.

Propagation of magnetoacoustic waves in stratified media

Propagation of magnetoacoustic waves in stratified media

The Effect of Finite Ion Larmor Radius on the Propagation of Magnetoacoustic Waves

The modified C.G.L. equation which includes the effect of the finite Larmor radius of the ion is applied to the study of hydromagnetic wave propagation in a collisionless plasma. The dispersion relation is given for a linearized system in which waves propagate in the direction normal to a magnetic field. Then the dispersive effects of the finite ion Larmor radius and the electron inertia are investigated in various cases. A precursor with an oscillatory structure appears in front of the main disturbance due to the former effect, and the latter gives rise to an oscillatory trail following behind.

Magneto-Acoustic Waves in a Plasma

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